Aircraft and other air vehicles may be subject to the formation of ice, such as upon the various leading edges, when exposed to icing conditions. Similarly, other aerodynamic surfaces, such as the blades of a wind turbine or a rotorcraft, may be subject to the formation of ice when exposed to icing conditions. Ice formation upon a leading edge of an aerodynamic surface may have an adverse effect upon the performance of the aircraft, rotorcraft, wind turbine or the like. As such, various ice protection systems have been developed to prevent or reduce the formation of ice, at least upon selected aerodynamic surfaces. In this regard, ice protection systems may heat a leading edge or other aerodynamic surface to a temperature above that suitable for ice formation in order to prevent or reduce ice formation.
With respect to aerodynamic surfaces formed of one or more thermally conductive layers of material, heating elements may be disposed within the aerodynamic surface with the heat generated by the heating elements propagating through the thermally conductive layers so as to heat the leading edge or other aerodynamic surface. For example, the heating element may be formed of a patterned metal layer with associated wiring that is embedded within a composite skin structure. While an embedded heating element may heat the leading edge, the embedded heating element may also heat other portions of the composite skin structure, such as the entire composite skin structure, thereby resulting in inefficiencies with respect to the heating. Additionally, an embedded heating element adds to the manufacturing costs of the aerodynamic surface and is not generally repairable.
Additionally, heating techniques that rely upon an embedded heating element may be insufficient, however, for aerodynamic surfaces that are not formed of thermally conductive materials. By way of example, the inlet lip surface of an engine nacelle may include an acoustic liner for attenuating inlet noise. In this regard, the inlet lip surface of an engine nacelle may be lined with an acoustic liner having a honeycomb core sandwiched between a perforated composite face sheet and a solid composite back sheet. An inlet lip surface of an engine nacelle may also include a porous metallic erosion barrier that overlies the porous composite face sheet in order to protect the acoustic liner.
Although the acoustic liner may prove effective for attenuating inlet noise, the acoustic liner and, in particular, the honeycomb core and the air contained within the cells of the honeycomb core may effectively thermally insulate the exterior surface, that is, the porous metallic erosion barrier, upon which ice may form from heat that may be generated by heating elements positioned within the inlet lip surface, such as heating elements positioned interior of the composite back sheet. As such, conventional heating methods may prove less effective than desired with respect to protecting an inlet lip surface having an acoustic panel from ice formation.
Another technique for restricting ice formation relies upon bleed air that has been heated and is recirculated to the inlet lip surface of the engine nacelle in order to heat the inlet lip surface. As will be apparent, ice protection systems that rely upon bleed air are not effective for electric aircraft or other systems that do not have bleed air available. Moreover, the generally static air within the cells of the honeycomb core of an acoustic liner may also insulate the exterior surface from the bleed air, thereby reducing the heat transfer rate to the exterior surface and/or increasing the amount of bleed air required to protect the exterior surface from ice formation.
Ice protection systems are being developed for aerodynamic surfaces that include acoustic liners. For example, a low-power electrical de-icing system utilizes embedded wire mesh heating elements disposed within a composite layer positioned near the exterior erosion barrier. The wire mesh heating elements of a low-power electrical de-icing system may disadvantageously increase the weight of the aerodynamic surface may consume a meaningful amount of power in order to support current flow through the entire wire mesh network, may be sensitive to external impacts upon the aerodynamic surface and may have increased maintenance costs. As a result of the power consumption, the power supply of the low-power electrical de-icing system may need to be sized in such manner that it also disadvantageously increases the weight and the cost of the aerodynamic surface. As another example, a transpiration flow anti-icing system is also being developed for titanium lip inlets having acoustic liners. A transpiration flow anti-icing system requires engine bleed air and is therefore inapplicable with respect to an electric aircraft or other system in which bleed air is unavailable. Also, since the upper temperature limit of the composite materials that are employed in acoustic liners is about 220° F. and since the bleed air temperatures are substantially higher, such as between about 800° F. and 1100° F., a transpiration flow anti-icing system may not be suitable for anti-icing an acoustically treated aerodynamic surface that employs composite materials.
The anti- or de-icing treatment of the other aerodynamic surfaces, such as the leading edges of wings and empennage and front or nose of an aircraft may also be subject to similar considerations and constraints. For instance, the leading edge of a wing that is built from composite materials and includes a metallic erosion shield may be sensitive to temperatures incurred during anti- or de-icing that exceed the thermal limits of the composite materials